1. Field of the Invention
The invention relates to ocular implants, and, in particular, to an implant used in the treatment of glaucoma.
2. Background
Intraocular pressure in the eye is maintained by the formation and drainage of aqueous, a clear, colorless fluid that fills the anterior and posterior chambers of the eye. Aqueous normally flows from the anterior chamber of the eye out through an aqueous outflow channel at a rate of 2 to 5 microliters per minute.
Glaucoma is a progressive disease of the eye characterized by a gradual increase of intraocular pressure. This increase in pressure is most commonly caused by stenosis or blockage of the aqueous outflow channel, resulting in excessive buildup of aqueous fluid in the eyeball. Other causes include increase in venous pressure outside the eye which is reflected back through the aqueous drainage channels and increased production of aqueous. In a "normal" eye, intraocular pressure ranges from 8 to 21 mm mercury. In an eye with glaucoma, this pressure can range between the so called normal pressures and pressures up to as much as 50 mm mercury. This increase in intraocular pressure produces gradual and permanent loss of vision in the afflicted eye.
Existing corrective methods for the treatment of glaucoma include drugs, surgery, and implants. Pharmacological treatment is prohibitively expensive to a large majority of glaucoma patients. In addition, many people afflicted with the disease live in remote or undeveloped retirement areas where the drugs are not readily accessible. The drugs used in the treatment, in particular steroids, often have undesirable side effects and many of the long-term effects resulting from prolonged use are not yet known.
Surgical procedures have been developed in an effort to treat victims of glaucoma. An iridectomy, removal of a portion of the iris, is often used in angle-closure glaucoma wherein there is an occlusion of the trabecular meshwork by iris contact. Removal of a piece of the iris then gives the aqueous free passage from the posterior to the anterior chambers in the eye. A trabeculotomy, opening the inner wall of Schlemm's canal, is often performed in cases of developmental or juvenile glaucoma so as to increase the outflow of the aqueous, thereby decreasing intraocular pressure. In adults, a trabeculotomy shunts fluid through a trap-door flap in the eye that performs a valve-like function for the first few weeks after surgery. While often successful, these surgical techniques possess inherent risks associated with invasive surgery on an already afflicted eye. Furthermore, the tissue of the eye can scar over this small area and the eye reverts to the pre-operative condition, thereby necessitating the need for further treatment.
Ocular implants are often used in long-term glaucoma treatment. One such implant is disclosed in U.S. Pat. No. 4,457,757 entitled "Device for Draining Aqueous Humor" and commercially available as the Molteno.TM. Seton Implant. The implant comprises a drainage tube connected to one or more rigid plate reservoirs. The reservoir plates conform to the curvature of the eye. A reservoir plate is placed under Tenon's capsule and sutured to the sclera.
The drainage tube is implanted into the anterior chamber through a scleral flap. A second plate can be passed under the superior rectus muscle and sutured to the sclera. At this point, the body will form scar tissue around these plates. Increased pressure causes the tissues above the plates to lift and form a bleb into which aqueous flows from the anterior chamber drains via the drainage tube. This type of implant is disadvantageous as the plates are formed of a rigid plastic which makes insertion beneath the eye tissue difficult and time-consuming.
UK Patent Application 2,160,778 entitled "Aqueous humor drainage device" discloses a similar type of implant device comprising a drainage tube and a drainage body. The tube is fixed to and opens directly onto a surface of the body. The device is sutured to the sclera of the eye and the tube positioned within the anterior chamber to provide outflow for the aqueous contained therein. The device further includes a pressure gradient limiting valve formed as a slit in the tube. However, when the device is implanted in the eye and is surrounded by aqueous humor, an extremely small flow rate of aqueous humor will pass through the slit, thus the slit in the tube functions not as a valve but as a flow restrictor. This type of device does not allow patent, i.e., open or two-way, flow through the drainage tube, and prevents retrograde aqueous flow into the anterior chamber. This device is considered a valved glaucoma implant because it has a permanent structure in the flow path obstructing the flow of aqueous from the anterior chamber. A non-valved device has a patent uninterrupted flow of aqueous to and from the anterior chamber, i.e., the implant has no permanent structure in the fluid flow path which obstructs the flow of the aqueous.
During the first few days after surgery and before scar tissue forms a bleb around the glaucoma implant device, it is possible for too much fluid to flow from the eye and create an extremely low to zero pressure in the eye. This low pressure could cause the eye to collapse and cause considerable and often permanent damage to the eye. It is desirable to provide a sealing mechanism that would prevent this pressure drop during the initial stages of the bleb formation and would enable a greater pressure release once the bleb has formed. Actual experience has shown that it is often necessary to apply a "purse-string" suture around the drainage tube which occludes drainage flow until the bleb has formed. Generally, the scar formation requires up to 8 weeks. After the scar is formed, the suture dissolves, but occasionally a second surgical procedure is necessary to cut the suture.
U.S. Pat. No. 4,750,901 issued to Molteno discloses a glaucoma implant with an elevated peripheral ridge, a subsidiary elevated ridge on the upper surface of the implant and a drainage tube which leads from the upper surface of the plate to the anterior chamber of the eye. The Molteno patent discloses that the tube enters the peripheral ridge to a position above the upper surface of the plate and the subsidiary ridge is located around the entrance of the tube. The subsidiary ridge is forced against Tenon's tissue, i.e., Tenon's capsule, to create an initial bleb cavity much smaller in area than the total bleb cavity. The Molteno patent discloses that the addition of the subsidiary ridge to the upper surface of the plate around the exit of the tube has the effect of providing a pressure sensitive one-way valve effect. The Molteno patent also discloses that, once the eye recovers from the operation, the increased production of the aqueous fluid by the eye raises the pressure in the eye and also within the small bleb cavity causing the overlying Tenon's capsule to be lifted slightly thereby allowing fluid to gain access to the entire bleb cavity.
In practice however, the Molteno device fails to provide an effective sealing surface with Tenon's capsule and the desired one-way valve effect does not occur. The main reason that the Molteno device is not functional as a one-way valve is that the tissue of Tenon's Capsule stretches so that there is little pressure against the subsidiary ridge to provide a sealed valve surface. In addition, Tenon's capsule is a thin layer of tissue and theoretically can only maintain the orbital tissue pressure of approximately 0 to 2 mm mercury to form a seal against the ridge on the implant. The orbital tissue pressure can not maintain the pressure within the eye of between 4 to 12 mm mercury for a normal eye and up to 20 mm mercury for patients that have severe glaucoma with the seal against the subsidiary ridge on the Molteno implant. Further, the tissue of Tenon's capsule is thin and has greater fluid permeability than other tissues in the eye, such as the sclera or conjunctiva, and undesired fluid flow passes though the tissue of Tenon's capsule. These forces alone or in combination are stronger than the weak seal provided by the Molteno device and the Molteno device cannot form a sufficient seal to create the desired one-way valve. Instead, the Molteno subsidiary ridge allows fluid to flow around it and advances almost directly into the full sized bleb. Thus, the Molteno device is not effective in preventing an initial large pressure drop in the eye in most cases and post-operative hypotony ensues.
Therefore, it would be desirable to provide a non-valved glaucoma implant device which results in a temporary flow restriction of fluid from exiting the anterior chamber of the eye and after sufficient time has elapsed creates a patent flow of fluid to/from the anterior chamber of the eye without requiring a second surgical operation.